3. THE APPROACH USED FOR MERCATOR
3.1 The Mercator aircraft
Mercator Figure 1 is a solar powered aircraft designed to fly at lower stratospheric altitude 15-20 km for very long duration,
carrying a 2.6 kg high resolution camera, called MEDUSA. It has a wingspan of approximately 22 m, and a total mass of
about 50 kg. Its wings are covered with a thin film solar array, which produces the electrical energy for propulsion, avionics
and the payload. During day time, excess energy is used to charge a string of batteries that provide power during night
time. Although these are the best to be found in the world, they cannot keep the aircraft at the highest altitude all night. So,
during night time, the aircraft slowly descends as well. 3.2
Early attempt
The Mercator project started in June 2006, before Belgian CAA had issued its Certification Specification. In absence of clear
rules, an safety case approach was proposed. The basis of that is to show that the proposed UAV flight has an equivalent level of
safety ELoS as other air traffic, and hence it should be allowed to fly. This safety case was a mathematical model, and
it proved that the Mercator aircraft poses significantly less threat to other aircraft or people on the ground than commercial
aircraft. However, important issues remained. The UAV was not
equipped with a transponder, and it did not carry navigation lights. Since the airframe is predominantly made of composites,
it reflects almost no RADAR waves, so the only way for Air Traffic Control to “see” the aircraft is via secondary RADAR a
transponder. At night, other aircraft pilots would be unable to see Mercator, with unacceptable collision risks as a
consequence, since Mercator does not have a “sense-and-avoid” system, nor the reaction capability to avoid other air traffic.
A workaround for the lack of a transponder was proposed: injecting Mercator’s position received via telemetry from the
on-board GPS receiver, but this involved unacceptable changes to the infrastructure of the ANSPs. Furthermore, GPS altitude is
not accepted in aviation, which is based on pressure altitude. So, an alternative approach had to be found.
3.3
A new aircraft
The early attempt had significant consequences: the Certification Specification for UAVs was issued, leading to
more clarity in what the authorities needed. A transponder and navigation lights were clearly required.
The impact on the Mercator aircraft is considerable: adding a transponder and lights adds about 1 kg to the aircraft’s mass,
and both draw power. This is a concern during night time, when batteries power the aircraft. In fact, the aircraft design had to be
modified to be able to carry the extra equipment and still meet the criteria set by Pegasus.
This new design proved its air worthiness in a flight trial in the USA, in July 2010, when the aircraft, called Zephyr, flew for
more than 14 days, breaking several world records. 3.4
Building confidence
Rather than coming up with a set of documents in the hope that they would be acceptable, we took the initiative to set up a
meeting with all relevant parties to kick off the new attempt. This meeting was attended by representatives from Belgian
CAA who were kind enough to host te meeting, Belgocontrol, ATCC, Eurocontrol as well as from QinetiQ aircraft designer
and builder and VITO. The outcome of this meeting was that a positive attitude was
shown by all, and also that this project was going to serve as a test for the procedures, because they had never been applied to
this case before. It was agreed that all would work together; the project was
placed at a high level of priority. 3.5
A launch location
The Mercator aircraft is hand launched 3-5 persons carry it above their heads and run for no more than 20 meters before the
aircraft lifts off, so there is no formal need for a runway or an airfield to start from. However, there are a number of rarely
used airfields in Belgium that offer facilities a hangar, communication lines, ... and are marked on aeronautical charts.
So it was decided to try to use an existing airfield, to minimise impact on the working methods of air traffic controllers.
The aircraft climbs slowly at low altitude, so sufficient distance from obstacles was required, which reduced the possibilities. In
the end, two alternatives both military, with very little traffic were selected and proposed to the other parties. One was
considered to be too close to Brussels airport, so the remaining option was then the baseline for operations: NATO reserve
airfield Bertrix designated EBBX. Operating on a military installation, even when it is only used
infrequently, needs to be approved by the Minister of Defense. We applied for an authorisation, and obtained it. From that
point onwards, support from all sections in Belgian Defense was strong. One important asset was that we could use the
services of the Meteo Wing, who are trained in weather predictions for aviation.
3.6
A stepwise approach
Now, we proposed to execute three flights, to test and validate the procedures, and to show that the Mercator flights were
indeed safe. Flight 1 was to take off at dawn and land at dusk so fly for
about 12 hours, carrying transponder and navigation lights, but no payload, flying to no more than 24 500 ft, so that it did not
interfere with Eurocontrol MUAC operations. Flight 2 would extend the duration to 24 hours take-off and
landing at dawn, with transponder and navigation lights, and climbing to more than 24 500 ft, hence involving Eurocontrol
as well. Flight 3 would fly for 3 consecutive days, and carry the
MEDUSA camera as well as transponder and navigation lights. Target altitude was over 60 000 ft during day time and more
than 35 000 ft at night. In a following meeting, Flight 1 was considered to be
unnecessary by ANSPs and Belgian CAA, so it was dropped. In parallel to supplying Belgian CAA with the technical
documents of the aircraft and crew, the ANSPs worked on the operational scenario of the flights. Closing a substantial part of
the air space to all other traffic did not seem to be a viable option to them. This was an advantage to the Pegasus project,
because it aims at providing remote sensing capability above Europe and other inhabited regions.
All flights use class C airspace, so separation between Mercator and other air traffic is done by ATC. For that reason, Belgian
CAA requires that at least one member of the Ground Control Station crew has an Instrument Rating. This ensures that
conversation to and from ATC is done in the manner used by all pilots.
ISPRS Zurich 2011 Workshop, 14-16 September 2011, Zurich, Switzerland
3
The result of the ANSPs collaboration was an operational method to create a dynamic buffer around the aircraft in 3
dimensions, into which other air traffic was not allowed. Only during ascent and decent, a part of the airspace was to be closed
to all other air traffic TRATSA UAV Bertrix on Figure 2.. Of course, only part of the Belgian airspace was available for this
exercise the Belgian part of TRA S2 Beauraing and TRA S5
Figure 2. Airspace available for Mercator Neufchateau in Figure 2., measuring about 55 km East-to-West
and North-to-South The flight crew would have to deal with this restriction and keep the aircraft within its containment area.
Figure 3. Real flown trajectories above 24 500 ft, on March 24, 2010, above the flight area for Mercator
3.7 Communication
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